Method to correct radial misposition of data tracks

ABSTRACT

A disk drive measures the radial misposition of tracks and repositions the tracks at regularly spaced intervals. Written in runout (WRO) in servo bursts is determined to calculate track spacing and squeeze among adjacent tracks. Tracks with improper spacing are repositioned by adding a squeeze correction term to the servo wedges. Thereafter, when the disk drive operates to store data, the servo bursts are used to calculate a position error signal (PES), and the squeeze correction term is combined with the PES to position the head at a proper track radius.

RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application No.60/299,140 filed on Jun. 18, 2001, entitled “Method to Correct RadialMisposition of Data Tracks”, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates in general to data storage, and inparticular to improving data storage reliability and efficacy in a diskdrive.

BACKGROUND OF THE INVENTION

Disk drives are well known in the computer art for providing secondarymass storage with random access. A disk drive comprises one or moremagnetic data storage disks rotated on a spindle by a spindle motorwithin an enclosed housing. A magnetic transducer head is placed on anactuator arm and positioned very closely to a corresponding disk surfaceby a slider suspended upon an air bearing. Servo information istypically written in servo sectors which are interleaved between datasectors or blocks. Servo information provides a servo controller withhead position information to enable a head positioner, such as a rotaryvoice coil motor (VCM), to move the actuator arm and therefore the headfrom track-to-track during random access track seeking operations, andto maintain the head in proper alignment with a track centerline duringtrack following operations when user data is written to or read from theavailable data sectors of the disk surface. As such, the servocontroller controls head positioning as the head is moved transverselyacross the tracks by the actuator arm, and maintains the head over aparticular track as the disk spins. The servo controller also controlsthe acceleration of the head which results from a force supplied by theVCM to the actuator arm.

The servo controller receives head position readings from the head. Thehead position is determined from the servo information written directlyonto the disk by e.g. a servo writer as part of the manufacturingprocess. The servo information may include the track number and indicatehow far the head is from the track centerline. That is, certaininformation on each track is reserved for indicating head position. Asthe head passes over the servo information, the track identification andposition indicators are read by the head and supplied to the servocontroller. The position indicators are at regularly spaced locations.Thus, the servo controller input from the head is not continuous but issampled.

The servo writer is typically stabilized on a large granite base tominimize unwanted vibration and employs an encoder (e.g. laserinterferometry) for position measurements. The servo writer suppliespower to the spindle motor for rotating the disk. The servo writer mayinclude a fixed head for writing a clock track onto one disk surface.The servo writer may also include a positioning system for moving apush-pin which extends through an opening in the disk drive housing andmechanically contacts the actuator arm. The positioning system uses thepush-pin to move the actuator arm and the head radially across the disk,and the head writes the track address and the servo information atseveral specified locations called servo wedges that extend radiallyacross the tracks. The servo wedges provide servo sectors for each trackon the disk.

The servo writer attempts to write the servo information in circulartracks that are evenly spaced across the disk surface. However, becauseof mechanical and electrical limitations, it not possible to obtain eventrack spacing. This track mispositioning is referred to as “squeeze”which limits the off-track read capability (OTRC) of the disk drive andcan cause encroachment (overwrite) leading to data loss. Existingmethods of correcting squeeze and encroachment are not effective whentwo adjacent tracks are radially positioned too close to or too far fromone another.

This problem is increasingly significant as track densities (measured intracks-per-inch (TPI)) are increased. Increased TPI makes track spacingerrors and the resulting squeeze more significant to data integrity.Existing methods of detecting squeeze include detecting data corruptiondue to encroachment. For example, during disk drive manufacturing, aflaw scan test identifies the disk locations which do not providereliable read and write operations. These disk locations are loggedwithin the disk drive and mapped out of the available customer dataarea. However, such techniques are time consuming and expensive.

There is, therefore, a need to efficiently detect squeeze during diskdrive manufacturing. There is also a need for correcting squeeze inorder to prevent encroachment and degradation in the OTRC of the diskdrive.

BRIEF SUMMARY OF THE INVENTION

The present invention satisfies these needs. In one embodiment, thepresent invention measures the radial misposition of the tracks andrepositions the tracks at regularly spaced intervals. This allows forhigher track densities and consequently higher capacity disk drives. Ina first step, written in runout (WRO) in servo pattern bursts written bya servo writer is measured and used to calculate track spacing andsqueeze among adjacent tracks. In one implementation, for each track,the WRO per servo wedge is determined based on a combination of servobursts at different radial distances from the track centerline indifferent disk revolutions. The measured WROs of the track servo wedgesare combined to obtain a squeeze value for the track relative to anadjacent track. The squeeze value indicates whether the track isproperly radially positioned.

The tracks with improper spacing, indicating they are radiallymispositioned (squeeze), are identified and effectively repositioned byadding a squeeze correction term to specific fields in the servo wedgesof the tracks to correct for the squeeze. Thereafter, when the diskdrive is operating, the servo bursts are used to calculate a positionerror signal (PES), and the squeeze correction term in the servo fieldsis combined with the PES to obtain an adjusted PES to position the headsuch that the reading/writing takes place at a proper track radius. Thistells the servo controller how to position the head to write at anadjusted track location (radius) that prevents squeeze.

In another version of the present invention, sectors of a track that areradially misplaced (squeeze) are identified, are logged within the diskdrive, and are mapped out of the available customer data area.Thereafter, the disk drive does not use the mapped out sectors for datatransfer.

The present invention efficiently detects track spacing and squeezeduring disk drive manufacturing. Further, the present invention correctsradial position of tracks with improper spacing, or maps out sectorswith improper spacing, in order to prevent encroachment and degradationin the OTRC of the disk drive.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become understood with reference to the followingdescription, appended claims and accompanying figures where:

FIG. 1 shows a diagrammatic example of a disk drive in which the presentinvention can be implemented;

FIG. 2 shows a diagrammatic example of a disk with several concentrictracks and a servo pattern;

FIG. 3 shows example servo bursts in servo wedges of the servo patternin FIG. 2;

FIG. 4 shows sample plots of a readback signal for the servo bursts inFIG. 3;

FIG. 5A shows example burst signal value plots for the servo bursts of atrack with proper track spacing (no squeeze);

FIG. 5B shows example burst signal value plots for the servo bursts of atrack with improper track spacing (squeeze);

FIG. 6A shows other example burst signal value plots for the servobursts of a track with proper track spacing;

FIG. 6B shows example burst difference signal value plots for the servobursts of FIG. 6A;

FIG. 6C shows example signal value plots for the PES and WRO for theservo bursts of FIG. 6B;

FIG. 7A shows other example burst signal value plots for the servobursts of a track with improper track spacing;

FIG. 7B shows example burst difference signal value plots for the servobursts of FIG. 7A;

FIG. 7C shows example signal value plots for the PES and WRO for theservo bursts of FIG. 7B;

FIG. 8A shows an example flowchart of track spacing measurement andcorrection;

FIG. 8B shows another example flowchart of track spacing measurement andcorrection;

FIG. 9 shows an example WRO plot of a track based on servo burst signalvalue differences where some sectors of the track have improper trackspacing;

FIG. 10A shows multiple track centerlines defined by example nominalburst placements without squeeze;

FIG. 10B shows an example burst placement where a burst seam ismisplaced and a track is squeezed;

FIG. 10C shows an example burst placement where a range of tracks aremisplaced; and

FIGS. 11A–D collectively show an example burst placement and RRO fieldsfor several tracks including RRO field stitching.

Like reference symbols and characters refer to like elements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram illustrating a disk drive 10 in which thepresent invention can be implemented. The disk drive 10 can be coupledto an external host computer 26 that uses the disk drive 10 as a massstorage device. The disk drive 10 includes functional blocks that do notnecessarily represent discrete hardware elements. For example, two ormore of the functional blocks can be implemented in firmware in a commondigital processor.

The disk drive 10 includes a data storage disk 12, a transducer head 14,an actuator arm 16, a voice coil motor (VCM) 18, a read/write channel20, an interface 22, a servo controller 24 and a drive controller 28.

The disk drive 10 receives read and/or write requests from the hostcomputer 26 and carries out the requests by performing data transfersbetween the disk 12 and the host computer 26. In a preferred embodiment,the disk drive 10 includes multiple disks 12 in a vertical stack withone head 14 for each operative disk surface. Typically, both surfaces ofeach disk 12 will be operative for storing user data and the disk drive10 will include two heads 14 for each disk 12. Single-sided diskarrangements can also be used.

The interface 22 provides an interface between the disk drive 10 and thehost computer 26. During read and write operations, the interface 22provides a communications path that includes data buffering between thehost computer 26 and the channel 20. In addition, the interface 22receives commands and requests from the host computer 26 and directsthem to the drive controller 28. The drive controller 28 then carriesout the commands by appropriately controlling the elements within thedisk drive 10.

The VCM 18 controllably positions the head 14 with respect to itscorresponding disk surface in response to a control signal generated bythe servo controller 24. The head 14 is coupled to the actuator arm 16and moves under the influence of the VCM 18. When performing a read orwrite operation, the drive controller 28 instructs the servo controller24 to move the head 14 to a target track on the disk 12 so that a datatransfer can take place. The servo controller 24 then generates acontrol signal to move the head 14 from a present location to theindicated target track during a seek operation.

Once the head 14 has arrived at the target track, the servo controller24 enters a track follow mode and maintains the head 14 in asubstantially centered position above the target track. The bulk of thedata transfer between the head 14 and the target track occurs during thetrack follow mode.

The channel 20 performs the data transformations necessary to providecommunication between the host computer 26 and the disk 12. For example,during a write operation, the channel 20 converts digital data receivedfrom the host computer 26 into an analog write current for the head 14.During a read operation, the channel 20 provides the datatransformations necessary for converting an analog read signal receivedfrom the head 14 into digital data that can be recognized by the hostcomputer 26. The channel 20 also separates out servo information read bythe head 14 and directs the servo information to the servo controller 24for positioning the head 14.

FIG. 2 shows the disk 12 with tracks 15 positioned from the innerdiameter (ID) to the outer diameter (OD) of the disk 12, and the head 14positioned over a track 15. After the disk drive 10 is assembled andservo written, during a flaw scan test in the manufacturing process thespacing of the tracks 15 is detected, and if the spacing of a track 15relative to the adjacent tracks 15 is not within a desirable range(squeeze) then each improperly spaced track 15, or each improperlyspaced sector of a track 15, is mapped out or repositioned. Byseparately measuring the track spacing, the marginal tracks 15 can bedetected independently of the performance of the head 14.

In one version of the present invention, the tracks 15 that areimproperly spaced (exhibit squeeze) are identified and mapped out of thecustomer data storage area. The improperly spaced tracks 15 and sectorsare logged and entered into a defect map of the disk drive 10. Inanother version, the tracks 15 that are improperly spaced are identifiedand repositioned for proper spacing to obtain concentric tracks 15 atspaced intervals without squeeze (essentially regularly spaced intervalswithout squeeze).

In either case, detecting the track spacing uses a combination of servobursts. As shown in FIG. 2, an improperly spaced or squeeze track 15 scan include DC squeeze where the radius 15 r of the track 15 s is toosmall or too large in relation to the adjacent tracks 15, leading tounevenly spaced tracks 15 on the surface of the disk 12. The radius ofthe track 15 s is too small, and the proper position of the track 15 sis shown by the dotted circle 15 p.

The track 15 can also include AC squeeze where only portions (e.g.,sectors) of the track 15 are written with improper spacing in relationto the adjacent tracks 15 such that the track 15 is perturbed from acircular shape but the average radial position of the track 15 isessentially correct. The present invention determines both the DCsqueeze and the AC squeeze.

Referring to FIGS. 2 and 3, the servo information includes servo burstpatterns 13 c in servo wedges 13 a that form essentially radial servospokes around the disk 12. Although the disk 12 is illustrated as havinga relatively small number of the tracks 15 and the servo wedges 13 a, itcan be appreciated that a typical disk includes a very large number oftracks and servo wedges.

FIG. 3 illustrates portions of the tracks 15 laid out linearly in adown-track 9 (circumferential) direction from left to right, and in across-track (radial) direction from top to bottom. Three centerlinesTn−1, Tn and Tn+1 of three tracks 15 are defined by multiple servowedges 13 a on each track 15. Each servo wedge 13 a begins with digitalinformation (e.g., AGC, sync mark, gray code, etc.) (not shown) andcircumferentially sequential, radially offset servo bursts 13 c whichprovide analog information to the servo controller 24 for positioningthe head 14. In this example, in each servo wedge 13 a, the servo bursts13 c are four staggered A, B, C and D bursts. The servo wedge 13 a caninclude further information following the servo bursts 13 c as describedbelow. Other numbers of servo bursts and offset configurations are alsopossible. In this example, the A, B bursts form a burst pair and the C,D bursts form another burst pair. The AC squeeze and the DC squeeze arenot to be confused with the A, B, C and D bursts or the burst seamsthereof.

During servo writing, the head 14 is positioned to write the A burst,then the head 14 is moved by ⅔ track width to write the B burst in anext revolution of the disk 12, thereby trimming off the bottom edge ofthe A burst and defining a burst seam (transition) 13 d between the Aburst and the B burst. Thus, the A burst is written in a firstrevolution of the disk 12, and then the A burst is trimmed when writingthe B burst in a second revolution of the disk 12, thereby creatingposition information in the second revolution of the disk 12 by thetrim/write operation. A different pair of revolutions of the disk 12create the position information for the C, D burst pair. The C burst iswritten first, and then in a different revolution of the disk 12 the Dburst is written, thereby trimming the edge of the C burst and forminganother burst seam 13 e between the C burst and the D burst. As such, ina pair of revolutions of the disk 12 the A, B burst pair and the burstseam 13 d are written, and in a different pair of revolutions of thedisk 12 the C, D burst pair and the burst seam 13 e are written.

The motion of the head 14 from the servo writer defines where the burstseams 13 d and 13 e occur. Since the head 14 has non-repeatable motion,the difference in position for the burst seam 13 d and the burst seam 13e captures the non-repeatable runout (NRRO) written into the A, B and C,D burst pairs as the AC squeeze. Therefore, NRRO is captured/writtenduring servo writing and is different from revolution-to-revolution ofthe disk 12, and the burst seams 13 d and 13 e are written with the ACsqueeze.

If the head 14 is placed at the burst seam 13 d, the readback signal ofthe head 14 includes half the signal value of the A burst and half thesignal value of the B burst. If the head 14 is shifted towards the Aburst, the magnitude of the A burst readback signal increases and themagnitude of the B burst readback signal decreases, thereby providingthe head 14 position information. The same readback signals and positioninformation apply to the C, D burst pair. The A, B and C, D burst pairsare shifted in position from each other by a fraction of the trackwidth, such as ⅓ track width in this example.

FIG. 4 shows sample readback signal plots 30, 32, 34 and 36 for the A,B, C, D bursts, respectively. The horizontal axis (X) indicates theradial track position and the vertical axis (Y) indicates the signalamplitude. In the description herein, the servo bursts and the servoburst readback signals are used interchangeably. As shown in FIG. 4,near track 1001 all the burst signals are in their linear range and thesignal values |A–B|=|C–D|. At this track position, the four burstcombination determines the head 14 position. For the head 14positioning, in one example, the signal value from the flux transitionsin the servo bursts induced to the head 14 are decoded by demodulatingthe induced head signals to form difference values (difference signals)including the A–B and C–D phases. The head 14 position information isdecoded by using combinations of the A–B burst phase and the C–D burstphase depending on the radial (cross-track) location of the head 14relative to the track centerline. The difference signals can be used incombination to obtain a PES for positioning the head 14 by the servocontroller 24. The PES represents the linear portions of the differencesignals indicating the direction of the head 14 movement for maintainingthe head 14 at the desired track position (e.g., at a track centerline,0.25 width from a track centerline, between two track centerlines,etc.).

The channel 20 includes a servo burst signal value detector circuit (notshown) which sequentially samples and holds analog burst signal valuesread from the A, B, C and D bursts by the read element of the head 14.The detector circuit extracts the signal values (e.g., peak signalvalues) of the A, B, C and D bursts read from each servo sector. Theburst signal values are used to generate the PES. The servo controller24 reads the PES and computes a new control setting after each servosector to drive the actuator arm 16 in a direction that reduces the PES.Other methods in addition to peak detection include a discrete fouriertransform (DFT) and are known by those skilled in the art.

The AC and DC squeeze values are functions of the WRO when the servobursts were written by the servo writer. Therefore, to determine thesqueeze, the track WRO is measured and the squeeze is determined basedon the WRO.

A version of measuring track spacing and squeeze between tracks based onWRO involves combining the signal values of the servo bursts in anorthogonal manner compared to the way they are combined for the PES. Theservo burst signal value combinations for the PES are proximate to thetrack centerline where the servo bursts yield a signal with high slopeproviding high sensitivity when the head 14 strays from the trackcenterline. The orthogonal combination signal of the burst signal valuesrepresenting the WRO is the converse of the PES and insensitive to thehead 14 stray. However, the WRO signal is very sensitive to trackspacing and therefore is a good indication of squeeze. Comparing the WROof a track to that of other tracks indicates whether the track's spacing(relative to the adjacent tracks) is higher or lower than average. Ahigher or lower than average WRO indicates squeeze.

FIG. 5A illustrates burst signals 30, 32, 34, 36 of the A, B, C and Dbursts, respectively, with the radial track position on the horizontalaxis (X) and the signal amplitude on the vertical axis (Y), for a trackwithout squeeze (e.g., normal A, B, C, D bursts, averaged over multiplerevolutions of the disk 12). At locations where the A, B, C and D burstsare active, the relative burst signal cross-over distance 38 isproportional to the track spacing. The cross-over distance 38 is thevertical distance between two burst cross-overs. For track 2117, thereis a B, C cross-over (intersection 38 a) near burst signal value 110 anda A, D cross-over (intersection 38 b) near burst signal value 475. InFIG. 5A, the track spacing for tracks 2117 to 2125 is essentiallyuniform and without squeeze.

FIG. 5B illustrates burst signals 30, 32, 34, 36 of the A, B, C and Dbursts, respectively, with the radial track position on the horizontalaxis (X) and the signal amplitude on the vertical axis (Y), for a trackwith squeeze. Track 672 shows reduction in the cross-over distance 38between the intersections 38 a and 38 b of the B, C and A, D burstsignal pairs, respectively, relative to the other cross-over distances38. The reduction in the cross-over distance 38 at track 672 indicatesthe WRO is outside the average WRO for track 672. Thus, track 672 is asqueeze track.

To determine squeeze for a track 15, the WRO for each servo wedge 13 ain the track 15 is determined at multiple positions (e.g., the trackcenterline, and ⅓ and ⅔ track width positions relative to the trackcenterline). Preferably the WRO measurement is essentially performed atevery track location where a valid position measurement can be obtained.In this example, the track centerlines are selected at new norm/quadpositions using the A, B, C and D bursts, and when the head 14 ispositioned on a track centerline the servo controller 24 drives|A–B|=|C−D| regardless of the values of |A–B| or |C–D|.

If within a given range of the tracks 15 the written track width isessentially constant and the track spacing is uniform as shown in FIG.5A, then the value |A–B| should be essentially constant over each track15 at the signal cross-overs. Similarly the value |C–D| should beessentially constant over each track 15 at the signal cross-overs. Asshown in FIG. 5B, reduced track spacing reduces the values |A–B| and|C–D| in the servo wedge 13 a at the signal cross-overs relative toother tracks 15.

In generating the PES, depending on the head 14 location relative to thetrack centerline, different track modes based on combinations of theburst difference values are used. Determining the PES and thecorresponding WRO based on the track mode for the four burst servopattern is based on the following relations:

PES1 = f[(A − B) − (C − D)] (1a) WRO1 = f[(A − B) + (C − D)] (1b) PES2 =f[−(A − B) − (C − D)] (2a) WRO2 = f[(A − B) − (C − D)] (2b) PES3 = f[−(A− B) + (C − D)] (3a) WRO3 = f[−(A − B) − (C − D)] (3b) PES4 = f[(A −B) + (C − D)] (4a) WRO4 = f[−(A − B) + (C − D)] (4b)where each PES and corresponding WRO is a function (f) of combinationsof the A–B and C–D values.

Referring back to FIG. 3, in the above relations, PES1 and WRO1correspond to the head 14 position at a track centerline (e.g., Tn−1).PES2 and WRO2 correspond to the head 14 position at ⅓ track width fromthe track centerline. PES3 and WRO3 correspond to the head 14 positionat ⅔ track width from the track centerline. And, PES4 and WRO4correspond to the head 14 position at the adjacent track centerline(e.g., Tn).

FIGS. 6A–6C and 7A–7C shows additional plots in which the horizontalaxis (X) indicates the radial track position and the vertical axis (Y)indicates the signal amplitude.

FIG. 6A shows plots of the signal values for the A, B, C and D burstsfor a track without squeeze. FIG. 6B shows plots of the burst differencevalues A–B and C–D, designated as 40 and 42, respectively. FIG. 6C showsplots of PES=[(A–B)−(C–D)]g and WRO=[(A–B)+(C–D)]g, designated as 44 and46, respectively. In one example, g is a constant determined by alinearizer algorithm (there are different values of two-burst andfour-burst calculations).

FIG. 7A shows plots of the signal values for the A, B, C and D burstsfor a track with squeeze. The track has squeeze since the burst seam 13d (between the A and B bursts) is misplaced by 10% track width. FIG. 7Bshows plots of the corresponding burst difference values A–B and C–D,designated as 40 and 42, respectively. FIG. 7C shows plots of thecorresponding PES=[(A–B)−(C–D)]g and WRO=[(A–B)+(C–D)]g. In FIG. 6C thenominal and acceptable WRO is 1.0 for a track without squeeze, whereasin FIG. 7C the shifted position of the burst seam 13 d causes the WRO todrop from 1.0 to 0.8, indicating squeeze. For simplicity of explanation,the constant g is not used in portions of the following description.

In one implementation, the DC squeeze (SQDC) can be determined based on4 WRO measurements as follows:SQDC=f[WRO1+2WRO2+2WRO3+WRO4]  (5)

Referring back to FIG. 5B, to determine the DC squeeze between track 672and the adjacent track 673, WRO1 is measured at track 672, WRO2 ismeasured at track 672 plus ⅓ track width, WRO3 is measured at track 672plus ⅔ track width, and WRO4 is measured at track 673. WRO is measuredfor all the servo wedges 13 a of tracks 672 and 673, and each servowedge 13 a requires measurements at ⅓ track steps between tracks 672 and673. Relation 5 is for a 3:1 servo pattern (i.e., three servo trackwriter steps per track, as shown in FIG. 3). Other servo patterns (e.g.,2:1) are variations on Relation 5 according to the present invention.Therefore, the scope of the present invention is not limited to theexample 3:1 servo pattern, the A, B, C and D bursts and the WRO and PESand squeeze relations described herein. As those skilled in the art willrecognize, the present invention is useful with servo patterns havingdifferent burst numbers and burst placements than the four burstexample.

Preferably, to determine the DC squeeze of a track 15, the WRO valuesfor each servo wedge 13 a in the track 15 are averaged. There is anoffset in the WRO measurement which is common to the tracks 15,represented as WRO_(—)AVE. The absolute value of the WRO signal valuedepends on the disk 12, the head 14 and the disk drive 10 electronics(e.g., channel 20, controllers 24 and 28, etc.), and as a result it isaveraged as WRO_(—)AVE. In one example, WRO_(—)AVE is determined overmany tracks 15 as a tracking weighted average of the measured cross-overdistances 38 (FIGS. 5A–5B) obtained by a moving average. At everymeasurement location a new WRO value is obtained, and the WRO value iscompared to a spacing threshold range. If the WRO value is within thethreshold range then the track spacing is acceptable and the WRO valueis included in the moving average. If the WRO value is not within thethreshold range (indicating squeeze such as at track 672) then thatsqueeze location is logged and the WRO value is not included in themoving average. WRO_(—)AVE is subtracted in the example DC squeezecalculation as follows:SQDC=(WRO1−WRO _(—)AVE)+2(WRO2−WRO _(—)AVE)+2(WRO3−WRO_(—)AVE)+(WRO4−WRO _(—)AVE)  (6)

At each servo wedge 13 a, the burst seams 13 d and 13 e are used todetermine the corresponding WRO value described above. In one version ofthe present invention, the DC squeeze is the average value of the runoutin all the servo wedges 13 a of a track 15.

Returning to the above example, to determine squeeze for track 672, theWRO measurements are retained in memory for all 256 servo wedges 13 aaround the track 672. The WRO measurements are made and retained fortrack 672, track 672 plus ⅓ track width, track 672 plus ⅔ track width,and track 673. Then the squeeze calculation above is performed over allthe servo wedges 13 a of track 672, and the squeeze values are averagedto obtain the average squeeze for track 672. In each servo wedge 13 a,the WRO for each burst seam is determined, and then Relation 6 is usedto obtain the DC squeeze for the servo wedges 13 a. The SQDC values areaveraged over all the servo wedges 13 a around track 672 to provide anaveraged value (a squeeze correction term that may be a constant).

The squeeze correction term is written into an RRO field following thebursts 13 c in each servo wedge 13 a of track 672 to correct forsqueeze. In one example, one RRO field corresponds to the read positionof the head 14 and another RRO field corresponds to the write positionof the head 14. When the disk drive 10 is operating, the servo bursts 13c are used to calculate the PES, and the squeeze correction term in theRRO field is combined with the PES to obtain an adjusted PES for theservo controller 24 to position the head 14 such that reading/writingtakes place at a proper track radius (essentially without squeeze). As aresult, the servo controller 24 positions the head 14 to write at anadjusted track location (radius) that prevents squeeze.

Referring back to FIG. 2, the squeeze correction terms are written intothe RRO fields in the servo wedges 13 a of the squeeze track 15 s.During the disk drive 10 operation, the servo controller 24 uses thesqueeze correction terms in the RRO fields to position the head 14 at agreater (or smaller) radius, and consequently effectively radiallyrepositions the track 15 s to the radial location at the dotted circle15 p to be essentially evenly spaced in relation to the adjacent tracks15 without squeeze.

As such, a squeeze track can be repositioned. The track radius can bedecreased or increased towards the ID or OD, respectively, of the disk12 up to e.g. about 15 to 20% track width, depending on the squeezecorrection terms placed into the RRO fields. Each RRO field may includeother correction terms, such as embedded runout correction terms and ACsqueeze correction terms, for circularizing a track 15 which includesperturbations (squeeze sectors) but has a generally proper averageradius.

FIG. 8A shows a flowchart of a track spacing measurement and correctionprocess. After the disk drive 10 is assembled and servo written (step200), a track spacing measurement to detect squeeze is commenced (step202). The servo controller 24 seeks to a next track 15 and determines ifthe track 15 can be followed (the servo controller 24 can remain withina track follow window for several sectors) (step 204). If the track 15can be followed, then the track spacing relative to an adjacent track 15is measured (step 206). The track spacing is used to determine whetherthe entire track 15 includes squeeze (DC squeeze) such that the trackspacing (radius) is too small or too large based on a spacing threshold(step 208). If the track radius is acceptable, then it is determined ifone or more sectors of the track 15 include squeeze (AC squeeze). Sectorspacing for the next sector is calculated using WRO measurements (step210). It is then determined if the sector spacing is acceptable (step212). If the sector spacing is unacceptable, either the sector is mappedout or a correction term is determined and written in the correspondingservo wedge 13 a to effectively correct the sector spacing (step 214).If another sector remains to be tested in the track 15 (step 218), thensteps 210–214 are repeated. If no other sectors remain to be tested inthe track 15 (step 218), then if other tracks 15 remain to be tested(step 220) then the servo controller 24 seeks to the next track 15 (step204).

After determining the AC squeeze for each sector, the amount and tracklocation of the squeeze can be logged. In one example, the AC squeezeinformation is placed in a self-test log, and the squeeze locations inthe log are stored in a sector defect list (map) in the disk drive 10.Thereafter, when the disk drive 10 is operating, the servo controller 24checks the defect list and skips writing to a defective (squeezed)location/sector.

If the track radius (spacing) is unacceptable (step 208), then acorrection term is determined and written in the servo wedges 13 a ofthe track 15 to effectively reposition the track 15 to the correctradius (step 222), and the process proceeds to step 220. In oneimplementation of steps 206, 208 and 222, if the track radius is notacceptable then the track 15 that is too close to or too far from theadjacent track 15 is effectively repositioned. To prevent encroachment,the radial position of the squeeze track 15 (the track 15 with incorrectradius) is corrected.

FIG. 8B shows a flowchart of another track spacing measurement andcorrection process. The process is best understood in conjunction withFIG. 3. To determine squeeze for track Tn, WRO1 in each servo wedge 13 ais measured at track centerline Tn in one revolution of the disk 12(step 300), then WRO2 in each servo wedge 13 a is measured at track Tn−⅓track width in another revolution of the disk 12 (step 302), then WRO3in each servo wedge 13 a is measured at track Tn−⅔ track width inanother revolution of the disk 12 (step 304) and then WRO4 in each servowedge 13 a is measured at track centerline Tn−1 in another revolution ofthe disk 12 (step 306). The 4 WRO measurements per servo wedge 13 aoccur in different revolutions of the disk 12 by moving the head 14 fromtrack Tn to Tn−1 in the increments of ⅓ track width per disk revolution.Although 4 WRO measurements at ⅓ track widths are used in the example,other numbers of WRO measurements at other fractional track widths arecontemplated by the present invention. From the WRO information, thesqueeze between the adjacent tracks Tn and Tn−1 is determined.

The DC squeeze (SQDC) in servo track units between tracks Tn and Tn−1can be determined based on the 4 WRO measurements by the followingrelations (step 308):SQDC(n)=f[WRO(n)+2WRO(n−⅓)+2WRO(n−⅔)+WRO(n−1)]  (7)

-   -   or with WRO_(—)AVE taken into account:        SQDC(n)=[WRO(n)−WRO _(—)AVE]+2[WRO(n−⅓)−WRO        _(—)AVE]+2[WRO(n−⅔)−WRO_(—)AVE]+[WRO(n−1)−WRO _(—)AVE]  (8)

Since the WRO measurements are made at every ⅓ track width, WRO(n)(e.g., WRO1) and WRO(n−1) (e.g., WRO4) are measured on the centerlinesof the adjacent tracks Tn and Tn−1, respectively, and WRO(n−⅓) (e.g.,WRO2) and WRO(n−⅔) (e.g., WRO3) are measured at ⅓ and ⅔ track width,respectively, from the track centerline Tn towards the track centerlineTn−1. WRO(n−⅓) and WRO(n−⅔) are selected at the norm/quad nulls betweentracks Tn and Tn−1. At the norm/quad null, two servo bursts are lowsignal value and two servo bursts are high signal value at thatlocation—e.g., A burst low, B burst high, C burst low and D burst high.The WRO(n), WRO(n−⅓), WRO(n−⅔) and WRO(n−1) measurements for the servowedges 13 a of tracks Tn and Tn−1 are made in consecutive revolutions ofthe disk 12 and combined to calculate the SQDC for track Tn. The WRO canbe measured on a sector, track or cylinder basis. Each WRO measurementis taken at a different revolution of the disk 12. Then the WRO data isaveraged around one track revolution to obtain the WRO for track Tn. TheWRO data can be averaged down a disk stack (with multiple disks 12) toobtain the WRO for a cylinder.

After the SQDC is calculated it is determined if there is unwantedsqueeze (step 310). In one example, a positive SQDC means tracks Tn andTn−1 are too close together, and a negative SQDC means tracks Tn andTn−1 are too far apart. In case the SQDC indicates radial misposition ofessentially the entire track Tn, that can be corrected by effectivelyrepositioning track Tn by determining a correction term and writing itin the track Tn servo wedges 13 a to effectively reposition track Tn bycounteracting the squeeze (step 312).

Then it is determined if there is another track to be tested (step 314).If not, the process is completed. Otherwise, the WRO for the next track(Tn−1) commences, and WRO4 of the previous track (Tn) is used as WRO1for the track (Tn−1) (step (316) and the process returns to step 302.Thus, after WRO measurements are made along a track, three revolutionsof the disk 12 are used to make three WRO measurements for the adjacenttrack.

In the example, four measurements of WRO (one per disk 12 revolution)are used for each track squeeze calculation. However, in measuringsqueeze in consecutive tracks 15, after squeeze is calculated for afirst track 15, the last WRO measurement of the first track 15 is usedas the first WRO measurement for the next track 15, thereby saving adisk 12 revolution. For example, after squeeze is calculated for thefirst track 15, a test scan of three revolutions of the disk 12 pertrack 15 is performed to measure WRO at a different position of the head14 relative to the centerline of the track 15. The head 14 is firstpositioned at the track centerline for one WRO measurement per servowedge 13 a in a first revolution of the disk 12, then the head 14 ismoved ⅓ track towards the ID of the disk 12 relative to the trackcenterline for a second WRO measurement per servo wedge 13 a in a secondrevolution of the disk 12, and then the head 14 is moved ⅓ track towardsthe ID of the disk 12 relative to the track centerline for a third WROmeasurement per servo wedge 13 a in a third revolution of the disk 12.The WRO information from the three revolutions of the disk 12 is used todetermine squeeze between the track and an adjacent track based on theSQ relations above.

The AC squeeze based on WRO is also proportional to the differences A–Band C–D.

FIG. 9 shows an example WRO time capture equivalent plot for a trackbased on the burst differences (A–B) and (C–D), with the servo-wedges 13a passing under the head 14 (or time) on the horizontal axis (X) and thesignal amplitude on the vertical axis (Y). This example shows 10% trackpeak-to-peak misposition at three track locations (peaks) due to ACsqueeze that was written into the servo bursts. Once the AC squeezelocations at the servo wedges 13 a are determined based on the WRO,appropriate action such as mapping out the corresponding data sectorscan be performed.

Rather than mapping out, the sector squeeze information can be used totrigger embedded repeatable runout correction (ERC) to correct the ACsqueeze. The ERC removes runout from a track, making it more circular.An example ERC is provided in copending U.S. application Ser. No.09/753,969 filed on Jan. 2, 2001, entitled “Method and Apparatus for theEnhancement of Embedded Runout Correction in a Disk Drive”, which isincorporated herein by reference. The ERC is written in RRO fields inthe servo wedges, and upon disk drive operation, the servo controlleruses the ERC to position the head to avoid perturbations.

In one example, to calculate squeeze, WRO is measured at 4 locations—onthe two adjacent tracks and the two norm/quad nulls between the adjacenttracks. The WRO at the norm/quad nulls contributes 2× to the squeeze atthis location.

For example, for (A–B)−(C–D), the PES and WRO are determined as:

-   -   PES=LinB1[(A–B)−(C–D)]/2¹¹, in servo track units    -   WRO=LinB1[(A–B)+(C–D)]/2¹¹, in servo track units

LinB1 compensates for burst signal value variations for the 4 servoburst calculations such that the linear position of the burst differencesignals is used in the PES calculations. The WRO values are used in theabove SQ relations, where WRO1 and WRO4 are measured on the adjacenttracks, and WRO2 and WRO3 are measured at the norm/quad nulls betweenthe tracks. The SQ values are the squeeze between the tracks. Thissqueeze is written as the squeeze correction terms in the RRO fields inthe servo wedges 13 a of the squeeze track 15 to correct for the radiussqueeze. In addition, one RRO field corresponds to the read position ofthe head 14 and another RRO field corresponds to the write position ofthe head 14.

FIGS. 10A–10C show tracks 400 to 403 with the track centerlines 100defined by the A, B, C and D bursts, a nominal read element 14 a 3 ofthe head 14, the PES for tracks 400 to 403, and the PES at differentpositions relative to track 1. Tracks 400 to 403 have a track spacing of12 as indicated by the division marks 1, 2, 3, 4 . . . 12 between theadjacent track centerlines 100.

FIG. 10A shows tracks 400 to 403 without squeeze.

FIG. 10B shows an example burst placement where burst seam 102 (betweenthe A and B bursts) is misplaced and track 401 is squeezed. The burstseam 102 in each servo wedge 13 a of track 401 is misplaced, and allother nearby burst seams are nominal as in FIG. 10A. The main failuremode is caused by the single burst seam 102 being misplaced. The trackcenterline 100 a of track 401 is closer than average to track 402 andfarther than average from track 400. A method for implementing thesqueeze correction term for repositioning track 401 based on themeasured DC squeeze includes moving only track 401 by the measuredsqueeze to the corrected track centerline 100 b of track 401.

Another method for implementing the squeeze correction term forrepositioning based on the measured DC squeeze uses localizedoptimization which corrects a track based upon the nearby tracks when aburst seam is misplaced. This is also applicable to servo writingdiscontinuities shown by example in FIG. 10C, where a range of trackcenterlines 100 such as for tracks 401 and 402 are squeezed compared tothe nominal track centerlines 100 in FIG. 10A. To correct for squeeze,the servo writing discontinuities are spread out over many tracks. Anexample process includes driving the average squeeze over a range oftracks to a reduced acceptable value by repositioning each track ofinterest (another technique uses a recursive filter).

More than one track may have to be repositioned to spread a singlesqueeze event over several tracks. In one example, severe squeezebetween two adjacent tracks is distributed among several tracks (e.g., 5or 10 tracks depending on squeeze amount) so that each corrected trackcenterline is spaced relative to the adjacent tracks where the severesqueeze is reduced, and there is essentially no squeeze (e.g., slightsqueeze without encroachment). The tracks are repositioned toredistribute the squeeze when there is increased TPI (localized bunchingof the tracks) followed by reduced TPI. Thus, there can be differentspacings between different tracks. The above relations correct trackradius (reposition tracks) in these situations as well. Further, thetrack repositioning (correcting the track centerline) can be distributedamong several squeeze tracks.

FIGS. 11A–D show tracks 500 to 506 with track centerlines 100 defined bythe A, B, C, D bursts in each servo wedge 13 a and the corresponding RROfield 106 following the servo bursts 13 c in each servo wedge 13 a.Tracks 500 to 506 have a track spacing of 12 as indicated by thedivision marks 1, 2, 3, 4 . . . 12 between the adjacent trackcenterlines 100. In each servo wedge 13 a of track 501, the A, B burstseam 102 and C, D burst seam 104 cause the track centerline 100 a oftrack 501 to be closer than average to track 502. As a result, the trackcenterline 100 a of track 501 is moved (repositioned) to the correctedtrack centerline 100 b by placing a squeeze correction term in thecorresponding RRO field 106 a. FIGS. 11A–D also show the PES for tracks501 to 506 based on track modes 1, 3, 5, 7 and a maximum read element 14a 1, minimum read element 14 a 2, nominal read element 14 a 3 andminimum write element 14 b of the head 14.

If it becomes necessary to reposition a track by more than 10% trackwidth, and each RRO field 106 is written in the nominal track positionfor correcting the track centerline 100, then the servo controller 14may have difficulty reading the RRO fields 106 since the track may berepositioned beyond the OTRC of the head 14. Therefore, it is desirableto stitch together multiple RRO fields 106 such that the squeezecorrection terms in the RRO fields 106 can be read at both the correctedand uncorrected track centerlines 100. In track 501, two RRO fields arewritten to effectively generate a wide RRO field. The squeeze correctionterm is written in a first RRO field 106 a at the original trackcenterline 100 a (before the correction is applied), and the squeezecorrection term is written again in a second (stitched) RRO field 106 bat the corrected track centerline 100 b. The RRO fields 106 a and 106 blineup in time (similar to servo 2 gray code) and effectively form awide RRO field for track 501.

During disk drive 10 operation, initially upon seeking to track 501 andreading the servo bursts 13 c in a servo wedge 13 a, the servocontroller 24 follows the original (uncorrected) track centerline 100 auntil it reads the squeeze correction term in the first RRO field 106 aand thereafter moves the head 14 to the corrected track centerline 100b. If the corrected track centerline 100 b is more than 10% track widthaway from the original uncorrected track centerline 100 a, then in thenext servo wedge 13 a the head 14 is too far away from the first RROfield 106 a in the next servo wedge 13 a to read the squeeze correctionterm therein. Instead, the servo controller 24 reads the squeezecorrection term in the second RRO field 106 b in that servo wedge 13 a.Therefore, the squeeze correction term can be read over a greater radialrange relative to track 501.

In one example, the track spacing measurement and radial mispositioncorrection process is stored in the utility zone of the disk 12 asassembly code for execution by the disk drive 10 electronics. In anotherexample, the process is implemented in firmware in the disk drive 10electronics.

In another example, the host computer 26 can be a test station todetermine track spacing and squeeze and to correct for mispositionedsectors or tracks. The squeeze detection and correction processes areperformed during disk drive 10 self test where other ERC is applied. Asthose skilled in the art appreciate, other implementations arecontemplated by the present invention.

The present invention has been described in considerable detail withreference to certain preferred versions thereof; however, other versionsare possible. Therefore, the spirit and scope of the appended claimsshould not be limited to the description of the preferred versionscontained herein.

1. A method of determining written in runout (WRO) in a disk drive,wherein the disk drive includes a data recording disk and a transducerhead, the disk includes a servo wedge and first and second tracks, theservo wedge extends radially across the tracks and include includescircumferentially sequential, radially offset servo bursts, the tracksare adjacent to one another, the first track has a track width, thefirst track includes a first track centerline, the second track includesa second track centerline, the track centerlines are separated from oneanother by a track spacing, and the head reads data from and writes datato the tracks, the method comprising the steps of: (a) positioning thehead at a first radial position relative to the tracks; (b) reading afirst combination of the servo bursts using the head at the first radialposition during a first revolution of the disk to generate firstreadback signals; (c) measuring first burst signals from the firstreadback signals; (d) obtaining a combination of the first burst signalsto generate a first burst phase; (e) positioning the head at a secondradial position relative to the tracks; (f) reading a second combinationof the servo bursts using the head at the second radial position duringa second revolution of the disk to generate second readback signals; (g)measuring second burst signals from the second readback signals; (h)obtaining a combination of the second burst signals to generate a secondburst phase; and (i) determining an WRO for the first track using theburst phases, wherein the WRO indicates the track spacing.
 2. The methodof claim 1, wherein the radial positions are radially spaced from oneanother by less than the track spacing.
 3. The method of claim 2,wherein the first radial position is closer to the first trackcenterline than to the second track centerline and the second radialposition is closer to the second track centerline than to the firsttrack centerline.
 4. The method of claim 3, wherein the first radialposition is the first track centerline and the second radial position isbetween the track centerlines.
 5. The method of claim 3, wherein thefirst radial position is between the track centerlines and the secondradial position is the second track centerline.
 6. The method of claim3, wherein the radial positions are between the track centerlines. 7.The method of claim 6, wherein the first radial position is ⅓ the trackwidth from the first track centerline and the second radial position is⅔ the track width from the first track centerline.
 8. The method ofclaim 6, wherein the first radial position is a norm/quad null betweenthe track centerlines and the second radial position is anothernorm/quad null between the track centerlines.
 9. The method of claim 1,wherein the first radial position is the first track centerline and thesecond radial position is the second track centerline.
 10. The method ofclaim 1, wherein the revolutions are consecutive revolutions.
 11. Themethod of claim 1, wherein the servo bursts include an A burst, a Bburst, a C burst and a D burst, and the WRO is determined using adifference in the burst signals of the A and B bursts (A–B) and adifference in the burst signals of the C and D bursts (C–D).
 12. Themethod of claim 1, wherein the radial positions are radially spaced fromone another by less than the track spacing, the first radial position iscloser to the first track centerline than to the second trackcenterline, the second radial position is closer to the second trackcenterline than to the first track centerline, the revolutions areconsecutive revolutions, the servo bursts include an A burst, a B burst,a C burst and a D burst, and the WRO is determined using a difference inthe burst signals of the A and B bursts (A–B) and a difference in theburst signals of the C and D bursts (C–D).
 13. A method of determiningwritten in runout (WRO) in a disk drive, wherein the disk drive includesa data recording disk and a transducer head, the disk includes a servowedge and first and second tracks, the servo wedge extends radiallyacross the tracks and include includes circumferentially sequential,radially offset servo bursts, the tracks are adjacent to one another,the first track has a track width, the first track includes a firsttrack centerline, the second track includes a second track centerline,the track centerlines are separated from one another by a track spacing,and the head reads data from and writes data to the tracks, the methodcomprising the steps of: (a) positioning the head at a first radialposition relative to the tracks; (b) reading a first combination of theservo bursts using the head at the first radial position during a firstrevolution of the disk to generate first readback signals; (c) measuringfirst burst signals from the first readback signals; (d) obtaining acombination of the first burst signals to generate first burst phases;(e) positioning the head at a second radial position relative to thetracks, wherein the second radial position is closer to the second trackcenterline than the first radial position is to the second trackcenterline; (f) reading a second combination of the servo bursts usingthe head at the second radial position during a second revolution of thedisk to generate second readback signals; (g) measuring second burstsignals from the second readback signals; (h) obtaining a combination ofthe second burst signals to generate second burst phases; (i)positioning the head at a third radial position relative to the tracks,wherein the third radial position is closer to the second trackcenterline than the second radial position is to the second trackcenterline; (j) reading a third combination of the servo bursts usingthe head at the third radial position during a third revolution of thedisk to generate third readback signals; (k) measuring third burstsignals from the third readback signals; (l) obtaining a combination ofthe third burst signals to generate third burst phases; (m) positioningthe head at a fourth radial position relative to the tracks, wherein thefourth radial position is closer to the second track centerline than thethird radial position is to the second track centerline; (n) reading afourth combination of the servo bursts using the head at the fourthradial position during a fourth revolution of the disk to generatefourth readback signals; (o) measuring fourth burst signals from thefourth readback signals; (p) obtaining a combination of the fourth burstsignals to generate fourth burst phases; and (q) determining an WRO forthe first track based on the first, second, third and fourth burstphases, wherein the WRO indicates the track spacing.
 14. The method ofclaim 13, wherein the first and second, second and third, and third andfourth radial positions are radially spaced from one another by lessthan the track width.
 15. The method of claim 14, wherein the first andsecond, second and third, and third and fourth radial positions areradially spaced from one another by ⅓ the track width.
 16. The method ofclaim 14, wherein the first radial position is the first trackcenterline, the second radial position is a norm/quad null between thecenterlines, the third radial position is another norm/quad null betweenthe centerlines and the fourth radial position is the second trackcenterline.
 17. The method of claim 13, wherein the servo bursts includean A burst, a B burst, a C burst and a D burst.
 18. The method of claim17, wherein the first, second, third and fourth burst phases areproportional to a difference in the burst signals of the A and B bursts(A–B) and to a difference in the burst signals of the C and D bursts(C–D).
 19. The method of claim 17, wherein: at the first radialposition, the WRO is a function of [(A–B)+(C–D)]; at the second radialposition, the WRO is a function of [(A–B)−(C–D)]; at the third radialposition, the WRO is a function of [-(A–B)−(C–D)]; and at the fourthradial position, the WRO is a function of [-(A–B)+(C–D)].
 20. The methodof claim 17, wherein a position error signal (PES) positions the headover the tracks, and at the first radial position, the PES is a functionof [(A–B)−(C–D)]; at the second radial position, the PES is a functionof [-(A–B)−(C–D)]; at the third radial position, the PES is a functionof [-(A–B)+(C–D)]; and at the fourth radial position, the PES is afunction of [(A–B)+(C–D)].
 21. The method of claim 17, wherein aposition error signal (PES) positions the head over the tracks, and atthe first radial position, the WRO is a function of [(A–B)+(C–D)] andthe PES is a function of [(A–B)−(C–D)]; at the second radial position,the WRO is a function of [(A–B)−(C–D)] and the PES is a function of[-(A–B)−(C–D)]; at the third radial position, the WRO is a function of[-(A–B)−(C–D)] and the PES is a function of [-(A–B)+(C–D)]; and at thefourth radial position, the WRO is a function of [-(A–B)+(C–D)] and thePES is a function of [(A–B)+(C–D)].
 22. The method of claim 13, whereinthe first, second, third and fourth revolutions are consecutiverevolutions.
 23. A method of determining written in runout (WRO) in adisk drive having a data recording disk and a transducer head, whereinthe disk includes concentric data tracks and servo wedges embeddedwithin the tracks, each of the servo wedges includes circumferentiallysequential, radially offset servo bursts, and the head is positionableover the tracks for recording and playback of data on the tracks, themethod comprising the steps of: (a) positioning the head relative to atrack; (b) reading the servo bursts in the servo wedges using the headto generate readback signals, wherein the head reads the servo bursts atmultiple positions relative to a track centerline of the track duringmultiple revolutions of the disk to generate the readback signals; (c)measuring burst signals of the servo bursts from the readback signals;(d) obtaining combinations of the burst signals to generate burstphases; and (e) determining an WRO for the track based on the burstphases, wherein the WRO indicates a track spacing of the track relativeto an adjacent track.
 24. The method of claim 23, wherein the WRO is afunction of the track spacing.
 25. The method of claim 23, wherein step(e) includes obtaining orthogonal combinations of the burst phases. 26.The method of claim 23, wherein the servo bursts in each servo wedgeinclude an A burst, a B burst, a C burst and a D burst.
 27. The methodof claim 26, wherein step (d) includes selecting pairs of the burstsignals and generating the burst phases from the pairs of the burstsignals.
 28. The method of claim 27, wherein in step (d) a first burstphase is proportional to a difference in the burst signals of the A andB bursts (A–B).
 29. The method of claim 28, wherein in step (d) a secondburst phase is proportional to a difference in the burst signals of theC and D bursts (C–D).
 30. The method of claim 29, wherein in step (e)the WRO is a function of the first and second burst phases.
 31. Themethod of claim 30, wherein in step (e) the WRO is based on a differentcombination of the burst phases depending on a position of the headrelative to the track centerline.
 32. The method of claim 31, wherein:for a first position of the head relative to the track centerline, theWRO is a function of [(A–B)+(C–D)]; for a second position of the headrelative to the track centerline, the WRO is a function of[(A–B)−(C−D)]; for a third position of the head relative to the trackcenterline, the WRO is a function of [-(A–B)−(C–D)]; and for a fourthposition of the head relative to the track centerline, the WRO is afunction of [-(A–B)+(C–D)].
 33. The method of claim 31, wherein aposition error signal (PES) is based on the burst signals to positionthe head over the track, and for a first position of the head relativeto the track centerline, the WRO is a function of [(A–B)+(C–D)] and thePES is a function of [(A–B)−(C–D)]; for a second position of the headrelative to the track centerline, the WRO is a function of [(A–B)−(C–D)]and the PES is a function of [-(A–B)−(C–D)]; for a third position of thehead relative to the track centerline, the WRO is a function of[-(A–B)−(C–D)] and the PES is a function of [-(A–B)+(C–D)]; and for afourth position of the head relative to the track centerline, the WRO isa function of [-(A–B)+(C–D)] and the PES is a function of [(A–B)+(C–D)].34. A method of determining track spacing in a disk drive having a datarecording disk and a transducer head, wherein the disk includesconcentric data tracks and servo wedges embedded within the tracks, eachof the servo wedges includes circumferentially sequential, radiallyoffset servo bursts, and the head is positionable over the tracks forrecording and playback of data on the tracks, the method comprising thesteps of: (a) positioning the head relative to a track; (b) reading theservo bursts in the servo wedges using the head to generate readbacksignals, wherein the head reads the servo bursts at multiple positionsrelative to a track centerline of the track during multiple revolutionsof the disk to generate the readback signals; (c) measuring burstsignals of the servo bursts from the readback signals; and (d)determining a track spacing of the track relative to an adjacent trackbased on combinations of the burst signals.
 35. The method of claim 34,wherein the combinations of the burst signals represent written inrunout (WRO) in the track.
 36. The method of claim 35, wherein step (d)includes: (i) selecting a plurality of the servo bursts in each servowedge and obtaining combinations of the burst signals of the selectedservo bursts to generate burst phases; and (ii) determining an WRO foreach servo wedge at the track based on the burst phases, wherein the WROis a function of the track spacing.
 37. The method of claim 36, whereinstep (d) includes performing steps (i) and (ii) for the servo wedgesover the multiple revolutions of the disk.
 38. The method of claim 37,wherein step (d) includes: performing steps (i) and (ii) for the servowedges over the multiple revolutions of the disk, wherein the head is ata different position relative to the track centerline for eachrevolution of the disk; and determining the track spacing as a functionof the WROs for the servo wedges.
 39. The method of claim 38, whereinthe servo bursts in each servo wedge include an A burst, a B burst, a Cburst and a D burst.
 40. The method of claim 39, wherein step (d)includes selecting pairs of the burst signals and generating the burstphases from the pairs of the burst signals.
 41. The method of claim 40,wherein in step (d) a first burst phase is proportional to a differencein the burst signals of the A and B bursts (A–B).
 42. The method ofclaim 41, wherein in step (d) a second burst phase is proportional to adifference in the burst signals of the C and D bursts (C–D).
 43. Themethod of claim 42, wherein in step (d) the VVRO is a function of thefirst and second burst phases.
 44. The method of claim 43, wherein instep (d) the WRO is based on a different combination of the burst phasesdepending on a position of the head relative to the track centerline.45. The method of claim 44, wherein: for a first position of the headrelative to the track centerline, a first WRO (WRO1) is a function of[(A–B)+(C–D)]; for a second position of the head relative to the trackcenterline, a second WRO (WRO2) is a function of [(A–B)−(C–D)]; for athird position of the head relative to the track centerline, a third WRO(WRO3) is a function of [-(A–B)−(C–D)]; and for a fourth position of thehead relative to the track centerline, a fourth WRO (WRO4) is a functionof [-(A–B)+(C–D)].
 46. The method of claim 45, wherein a position errorsignal (PES) is based on the burst signals to position the head over thetrack, and for the first position of the head relative to the trackcenterline, the PES is a function of [(A–B)−(C–D)]; for the secondposition of the head relative to the track centerline, the PES is afunction of [-(A–B)−(C–D)]; for the third position of the head relativeto the track centerline, the PES is a function of [-(A–B+(C–D)]; and forthe fourth position of the head relative to the track centerline, thePES is a function of [(A–B)+(C–D)].
 47. The method of claim 45, whereinin step (d) the track spacing is a function of WRO1, WRO2, WRO3 andWRO4.
 48. The method of claim 47, wherein in step (d) the track spacingis a function of [WRO1+2WRO2+2WRO3+WRO4].
 49. The method of claim 43,wherein in step (d), for each track n, the track spacing (SQ(n)) is afunction (f) according to the relation:SQ(n)=f[WRO(n)+2WRO(n−⅓)+2WRO(n−⅔)+WRO(n−1)] and the WRO measurementsare made at every ⅓ track width per revolution of the disk, WRO(n) andWRO(n−1) are measured at track centerlines of the adjacent tracks n andn−1, respectively, WRO(n−⅓) and WRO(n−⅔) are measured at ⅓ and ⅔ trackwidth from the track centerline of the track n, and the WRO(n),WRO(n−⅓), WRO(n−⅔) and WRO(n−1) are measured in consecutive revolutionsof the disk.
 50. A method of determining written in runout (WRO) in adisk drive having a data recording disk and a transducer head, whereinthe disk includes concentric data tracks and servo wedges embeddedwithin the tracks, each of the servo wedges includes circumferentiallysequential, radially offset servo bursts, and the head is positionableover the tracks for recording and playback of data on the tracks, themethod comprising the steps of: (a) positioning the head relative to atrack; (b) reading the servo bursts in the servo wedges using the headto generate readback signals, wherein the servo bursts in the servowedges include an A burst, a B burst, a C burst and a D burst; (c)measuring burst signals of the servo bursts from the readback signals;(d) obtaining combinations of the burst signals to generate burstphases, including selecting pairs of the burst signals and generatingthe burst phases from the pairs of the burst signals, wherein a firstburst phase is proportional to a difference in the burst signals of theA and B bursts (A–B), and a second burst phase is proportional to adifference in the burst signals of the C and D bursts (C–D); and (e)determining an WRO for the track based on the burst phases, wherein theWRO indicates a track spacing of the track relative to an adjacenttrack, the WRO is a function of the first and second burst phases, theWRO is based on a different combination of the burst phases depending ona position of the head relative to a track centerline of the track, anda position error signal (PES) is based on the burst signals to positionthe head over the track, and for a first position of the head relativeto the track centerline, the WRO is a function of [(A–B)+(C–D)] and thePES is a function of [(A–B)−(C–D)]; for a second position of the headrelative to the track centerline, the WRO is a function of [(A–B)−(C–D)]and the PES is a function of [-(A–B)−(C–D)]; for a third position of thehead relative to the track centerline, the WRO is a function of[-(A–B)−(C–D)] and the PES is a function of [-(A–B)+(C–D)]; and for afourth position of the head relative to the track centerline, the WRO isa function of [-(A–B)+(C–D)] and the PES is a function of [(A–B)+(C–D)].51. A method of determining track spacing in a disk drive having a datarecording disk and a transducer head, wherein the disk includesconcentric data tracks and servo wedges embedded within the tracks, eachof the servo wedges includes circumferentially sequential, radiallyoffset servo bursts, and the head is positionable over the tracks forrecording and playback of data on the tracks, the method comprising thesteps of: (a) positioning the head relative to a track; (b) reading theservo bursts in the servo wedges using the head to generate readbacksignals; (c) measuring burst signals of the servo bursts from thereadback signals; and (d) determining a track spacing of the trackrelative to an adjacent track based on combinations of the burstsignals, wherein the combinations of the burst signals represent writtenin runout (WRO) in the track, including (i) selecting a plurality of theservo bursts in each servo wedge and obtaining combinations of the burstsignals of the selected servo bursts to generate burst phases; (ii)determining an WRO for each servo wedge at the track based on the burstphases, wherein the WRO is a function of the track spacing; performingsteps (i) and (ii) for the servo wedges over multiple revolutions of thedisk, wherein the head is at a different position relative to a trackcenterline of the track for each revolution of the disk; and determiningthe track spacing as a function of the WROs for the servo wedges at thetrack.
 52. The method of claim 51, wherein the servo bursts in eachservo wedge include an A burst, a B burst, a C burst and a D burst. 53.The method of claim 52, wherein step (d) includes selecting pairs of theburst signals and generating the burst phases from the pairs of theburst signals.
 54. The method of claim 53, wherein in step (d) a firstburst phase is proportional to a difference in the burst signals of theA and B bursts (A–B).
 55. The method of claim 54, wherein in step (d) asecond burst phase is proportional to a difference in the burst signalsof the C and D bursts (C–D).
 56. The method of claim 55, wherein in step(d) the WRO is a function of the first and second burst phases.
 57. Themethod of claim 56, wherein in step (d) the WRO is based on a differentcombination of the burst phases depending on a position of the headrelative to the track centerline.
 58. The method of claim 57, wherein:for a first position of the head relative to the track centerline, afirst WRO (WRO1) is a function of [(A–B)+(C–D)]; for a second positionof the head relative to the track centerline, a second WRO (WRO2) is afunction of [-(A–B)−(C–D)]; for a third position of the head relative tothe track centerline, a third WRO (WRO3) is a function of[-(A–B)−(C–D)]; and for a fourth position of the head relative to thetrack centerline, a fourth WRO (WRO4) is a function of [-(A–B)+(C–D)].59. The method of claim 58, wherein a position error signal (PES) isbased on the burst signals to position the head over the track, and forthe first position of the head relative to the track centerline, the PESis a function of [(A–B)−(C–D)]; for the second position of the headrelative to the track centerline, the PES is a function of[-(A–B)−(C–D)]; for the third position of the head relative to the trackcenterline, the PES is a function of [-(A–B+(C–D)]; and for the fourthposition of the head relative to the track centerline, the PES is afunction of [(A–B)+(C–D)].
 60. The method of claim 58, wherein in step(d) the track spacing is a function of WRO1, WRO2, WRO3 and WRO4. 61.The method of claim 60, wherein in step (d) the track spacing is afunction of [WRO1+2WRO2+2WRO3+WRO4].
 62. The method of claim 56, whereinin step (d), for each track n, the track spacing (SQ(n)) is a function(f) according to the relation:SQ(n)=f[WRO(n)+2WRO(n−⅓)+2WRO(n−⅔)+WRO(n−1)] and the WRO measurementsare made at every ⅓ track width per revolution of the disk, WRO(n) andWRO(n−1) are measured at track centerlines of the adjacent tracks n andn−1, respectively, WRO(n−⅓) and WRO(n−⅔) are measured at ⅓ and ⅔ trackwidth from the track centerline of the track n, and the WRO(n),WRO(n−⅓), WRO(n−⅔) and WRO(n−1) are measured in consecutive revolutionsof the disk.